Surface acoustic wave (SAW) devices are widely used as band-pass filters or resonators. Surface acoustic wave (SAW) devices are electronic components that generate guided acoustic waves along the surface of the device. Any changes to the characteristics of the propagation path affect the velocity, phase or amplitude of the acoustic waves propagating along the surface of the device. These changes can easily be measured. The changes in frequency, phase or amplitude can be correlated to a physical quantity such as temperature, pressure or strain, or the detection of the presence of a specific gas. Thus, the device can be used as a sensor.
SAW sensors are very sensitive because the propagating acoustic wave has its energy concentrated close to the device surface. SAW devices are typically fabricated on single crystal anisotropic substrates that are also piezoelectric, such as quartz. A piezoelectric material produces electrical charges when it is subjected to mechanical stress. This phenomenon is reversible. A SAW sensor used to measure temperature, pressure, strain or the presence of a gas, typically includes a pair of spaced apart interdigital electrodes formed by a metal and disposed on the surface of the substrate. The interdigital electrode pair creates mechanical stress in the substrate when an electric field is applied. The (oscillatory) electric field creates a mechanical wave that propagates along the surface of the substrate. A second pair of interdigital electrodes converts the received mechanical wave back into an electric signal that is then compared to the original signal.
One of the difficulties of achieving acceptable performance parameters with SAW sensors is that quartz undergoes an α to β transition at about 570° C. and loses its piezoelectric properties. Also, aluminum (Al), the most widely used metallization for SAW electrodes becomes soft when the temperature exceeds a few hundred degrees and actually melts at 660° C. For extended temperature ranges materials other than quartz have to be used. Materials such as LiNbO3, materials from the LGX family of crystals or gallium phosphate can be used to extend the temperature range.
Another difficulty with SAW sensors is the fact that they cannot easily differentiate between different physical parameters. For example, a typical SAW sensor cannot easily distinguish between temperature and strain or temperature and pressure. Various physical parameters influence the propagation properties of a mechanical wave and the sensor cannot distinguish among them.